1. Field of the Present Invention
The present invention relates to an illumination optical system employing an excimer laser as a light source and using a beam expander to magnify or reshape a laser beam from the excimer laser, and more particularly, to an illumination optical system used for semiconductor device manufacturing equipment, and a semiconductor device manufacturing method using the illumination optical system.
2. Discussion of the Related Art
In recent years, semiconductor circuit patterns have been decreasing in size in the manufacture of semiconductors or semiconductor chip-mounted boards. Accordingly, semiconductor device manufacturing equipment used to print these circuit patterns has been required to exhibit an ever-higher resolution. In addition, illumination optical systems employing excimer lasers as light sources that generate high output ultraviolet beams at shorter wavelengths have replaced mercury lamps, which were conventionally used as ultraviolet beam sources. These optical systems have been developed for transferring circuit patterns onto a wafer by photolithographic exposure. An illumination optical system that uses the excimer laser as a light source is an afocal optical system that includes a first group of lenses with positive refracting power, a second group of lenses with negative refracting power, and a third group of lenses with positive refracting power, arranged in that order from a light source. Such a conventional system is disclosed in a Japanese Patent Laid-Open Publication No. 01-96929. An illumination optical system using an afocal optical system that includes a first group of lenses with positive refracting power, a second group of lenses with positive refracting power, and a third group of lenses with positive refracting power, arranged in that order from a light source, is also known.
FIGS. 4(a) and 4(b) are schematic diagrams of an afocal optical system used in a conventional illumination optical system. FIG. 4(a) represents a schematic diagram at a relatively low magnification, and FIG. 4(b) represents a schematic diagram at a relatively high magnification. The afocal optical system of this conventional example comprises a first group of lenses L.sub.1 with positive refracting power, a second group of lenses L.sub.2 with negative refracting power, and a third group of lenses L.sub.3 with positive refracting power, arranged in that order from a light source. A parallel laser beam from the excimer laser, not shown in FIGS. 4(a) or 4(b), enters the first group of lenses L.sub.1 from the left side of the drawings and is focused at a focal point F.sub.1 by the first group of lenses L.sub.1, since the first group of lenses L.sub.1 exhibits positive refracting power. The laser beam then enters the second group of lenses L.sub.2, located in the vicinity of the focal point F.sub.1 after the first group of lenses L.sub.1. The beam emitted is then converted into a parallel beam again by the third group of lenses L.sub.3.
The afocal optical system used in the conventional illumination optical system has an advantage in that magnification can be changed significantly by slightly moving the second group of lenses L.sub.2, since the second group of lenses L.sub.2 is located in the vicinity of the focal point F.sub.1 after the first group of lenses L.sub.1. At the same time, however, a focus point exists near the neighborhood of the second group of lenses L.sub.2, which causes impurities in the atmosphere to accumulate on lens surfaces, thereby reducing the transmissivity of the lenses and heating the lenses, ultimately deteriorating lens material life drastically. In some cases, the lenses may actually break.